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POWER GENERATION AND TRANSMISSION 33
Including all types, stacks cost in normal times from $1.50 to $4 per boiler horsepower in large sizes.
Mechanical Draft.—This responds to changing loads and varying atmospheric conditions as the chimney can not, and permits of the use of fuels which could not be burned under any practicable chimney.
Mechanical draft may be afforded by a steam jet, by blast fans (forced draft) discharging to the ashpit, or by suction fans (induced draft) placed between the boiler and the short stack. Steam jets, whether of the suction or pressure type, are wasteful of steam, though cheap to install. They will not give very high drafts, but tend to prevent clinkering.
Forced draft fans should be used in connection with adequate stacks, so that a blowpipe action on the brickwork may be avoided. Careful regulation is necessary. Induced draft fans must be much larger (the volume of gases handled is greater) and are more costly to maintain because they handle hot and gritty gases.
Size of Fans.—In a given fan in service, the volume delivered varies directly as the speed (revolutions per minute), the pressure (draft) as the square of the speed and the power consumed as the cube of the speed. Hence large fans, though of greater first cost, are cheaper to operate. For similar fans operating at fixed peripheral speed (and therefore fixed draft) the volume and power consumed vary as the square of the diameter. A fan should be selected which at a reasonable revolutions per minute (generally under 275) will give the peripheral speed necessitated by the imposed draft.
FUELS AND FURNACES
Classification of Coals.—Coals obtained west of Pittsburgh are generally of inferior quality. Coal may be classified according to either the 'proximate or the ultimate analysis. Thus Frazer's classification:
: SEMI- SEMI- Brru-
KIND OF COAL ANTHRACITE ANTHRACITE BITUMINOUS MINOTTS
Ratio fixed carbon: volatile: 100 to 12 12 to 8 8 to 5 5 to 0
Also Campbell's classification, based on the ultimate analysis:
ANTHRA- SEMI- SEMI- BITU-
KIND OF COAL CITE ANTHRACITE BITUMINOUS MINOUS LIGNITE PEAT WOOD
Ratio C:H: above 26 23 to 26 20 to 23 11.2 to 20 9.3 to 11.2 below 9. 3 7.2
Heat Value.—The heat values of coals are "known from calorimetric determinations. The A. S. M. E. formula, based on the ultimate analysis, gives
B.t.u. per pound = 14,600C + 62,000 (H - ^) + 4,OOOS,
where C, H, 0 and S are the respective weights of carbon, hydrogen, oxygen and sulphur in 1 Ib. of fuel. Where the proximate analysis is given, Lucke proposes
B.t.u. per pound = 14,544F + 27,0007 fl - p-^—
I y + C
WHere F and 7 denote the weights of fixed carbon and volatile in 1 Ib. of fuel. Soft coals have various local classifications, such as the several sizes of "nut" and "screenings." In general, such coals are graded as "lump," "run-of-mine" and "slack," the latter either being the fine, weathered product resulting from exposure of the others, or the finer part of the "run-of-mine."